Demographic changes can result in alterations of social structure and behavior which in turn may affect survival, reproduction, or movement and may influence how populations respond to changes in the environment. In 2013, 52 Atlantic spotted dolphins (Stenella frontalis) left Little Bahama Bank (LBB) and immigrated into a resident group of spotted dolphins on Great Bahama Bank (GBB). Initially there was little interaction and two distinct social clusters delineated by residency status. Here we describe the social structure of the dolphins on GBB between 2015–2020. Analyses were conducted with SOCPROG 2.9 and coefficients of association were nonrandom. The number of associations between LBB and GBB individuals increased, and standardized lagged association rates indicated continued associations over time. Modularity analysis showed one community without clear social clusters, although there were preferential associations within the original communities. Multiple Regression Quadratic Assignment Procedure indicated that sex, age, and residency status were good predictor variables that explained the patterns of the association indices. Male alliances were present between LBB, GBB, and LBB/GBB males. New and old relationships are influencing the social restructuring of this community. This study furthers our understanding of how demographic changes can shape the social structure of a population.

With the exception of primates, detailed interspecies behavioral studies are rare in mammalian species and for cetaceans, most are anecdotal descriptions. This study is the first long-term study on interspecies associations of regularly interacting groups. In the Bahamas Atlantic bottlenose (Tursiops truncatus) and spotted dolphins (Stenella frontalis) regularly form mixed species encounters (MSE). Both species show strong site fidelity with high resighting rates. During MSE, the majority (>65%) of spotted dolphins (especially males) were continually resighted; however bottlenose dolphins had comparatively low resighting rates (<17%). During MSE group size was significantly larger for spotted dolphins (13.0 ± 11.0) than bottlenose dolphins (4.8 ± 3.5) (F = 93.803, df = 1, P < 0.001). This difference was largest during aggressive encounters, due to the increased spotted dolphin group size (t-test, t = 4.75, df = 184, P

The majority of cetacean research pertaining to the western North Atlantic Ocean has focused on the waters between North Carolina in the United States and the Scotian Shelf in Canada; little is known about cetacean occurrence and distribution in the waters off southeast Florida (FL) where the subject study was conducted. Our study describes opportunistic, ship-based sightings of cetaceans during 1989-2006 in nearshore and offshore waters located in the Gulf Stream between the Bahamas and Palm Beach, FL. Nine species were observed during 60 sightings. For two of the documented species (false killer whale, Pseudorca crassidens, and Fraser’s dolphins, Lagenodelphis hosei), very little existing information was available with respect to sightings and distribution in the study area. The other seven species were observed in waters south of their documented distributions, based on sightings data from dedicated surveys conducted along the US East Coast, but which only extended to central FL. We documented distinctive physical attributes of offshore ecotype bottlenose dolphins (Tursiops truncatus) and described foraging behavior of false killer whales. Our opportunistic observations highlight the importance of conducting regular surveys in this little-researched region. Data gathered during our study may have important implications for cetacean stock assessments and conservation strategies.

Long-term social structure data on small delphinids is lacking for most species except the bottlenose dolphin. This study describes the long-term social structure of one community of Atlantic spotted dolphins, Stenella frontalis, divided into three social clusters. Data from 12 yr were analyzed using SOCPROG 2.3. Coefficients of association (CoA) were calculated using the half-weight index. The overall mean community CoA ranged from 0.09 to 0.12. Temporal analyses and mantel tests revealed significant differences between sex class associations due to high male-male CoA (0.12–0.23) compared to female-female and mixed sex CoA (0.08–0.10). Female associations were strongly influenced by reproductive status, calf care, and social familiarity, but not by age class. Male associations were strongly influenced by age, access to females, and alliance formation. Males showed two levels of alliance formation, long-term first order pairs/trios (CoA 0.70–1.00) and shorter-term second order alliances between two or more first order alliances (CoA 0.45–0.69), and a possible third level during interspecies interactions. Mating strategies, sex, and cluster formation shaped the social structure in this spotted dolphin community. Similar to many bottlenose dolphin studies, long-term affiliations for spotted dolphins were correlated with age, sex, and reproductive status.

Extreme environmental events and demographic changes can have variable effects on the social structure of animal populations. This study compared the social structure of a community of Atlantic spotted dolphins in the Bahamas before and after two hurricanes. Approximately 36% of the individuals were lost, with no subsequent increase in immigration. The majority of the social structure characteristics were consistent with results from a long-term study covering the previous 12 yr, including community structure with definitive social clusters, sex preferences and overall association patterns. However some changes occurred, though still constrained within sex preferences. Posthurricane there was a decrease in social differentiation and increased cohesion within clusters and across age class. Males retained or made new first order alliances, however, only one second order alliance was evident, revealing a simplified alliance structure. Juvenile individuals made alliance level associations, unprecedented from long-term analysis. Although other studies have shown stark restructuring, this study showed that less drastic changes within overall social structure stability can occur. Persistence and evolutionary changes in populations through environmental and/or demographic perturbations may depend on the social structure of a population or community. Understanding the processes involved in social development is paramount for conservation of diverse populations.

In the Bahamas, interspecific groups of Atlantic spotted dolphins, Stenella frontalis, and bottlenose dolphins, Tursiops truncatus, have been observed underwater since 1985 on Little Bahama Bank. Mixed-species groups engage in associative behaviors and aggression on a regular basis. Because of their complex cognitive behaviors and large brain encephalization, dolphins are likely capable of complex social interactions, even between species.Between 1993-2003, 177 Mixed-Species Encounters (MSE) were categorized by the age class of male spotted dolphins, the ratio of spotted dolphins to bottlenose dolphins, behavior as Associative (traveling, babysitting, play) or Aggressive (chases, mounting, head to heads) and by directionality of sexual behavior. The majority (68%) of MSE involved adult spotted dolphin. Associative behaviors were observed more than aggressive behaviors in groups where no adult male spotted dolphin, only male calves, or male juvenile spotted dolphins were present. Aggressive behaviors were observed more frequently than associative behaviors in adult male spotted dolphin groups. When groups were unbalanced in favor of one species or the other, differences in social interactions occurred. Male spotted dolphins were never observed attempting to mount male bottlenose dolphin although they chased them. Despite the larger ratio of male spotted dolphins to bottlenose dolphins during MSE, directionality of male-to-male sexual contact was primarily one-way. Male bottlenose dolphin mounted and copulated with male spotted dolphins but not the reverse. Opportunities for cross-species mating and hybridization clearly occurred. Male bottlenose dolphins copulated with female spotted dolphins and male spotted dolphins copulated with female bottlenose dolphins. These sympatric dolphins in the Bahamas have a complex and dynamic relationship that varies with sex and age and revolves around potential reproductive isolation issues.

Fission-fusion dynamics typical of many delphinid populations allow for a variety of social grouping patterns. Identifying these groupings is crucial before conducting a detailed social structure analysis. This study analyzed the structure of a population of Bahamian spotted dolphins, Stenella frontalis. Through long-term observations and preliminary analysis, three clusters were defined: Northern, Central, and Southern. To quantitatively investigate these delineations, we conducted analysis on 12 yr of sighting data using SOCPROG 2.3. Coefficients of association (CoA) were calculated using the half-weight index, with individuals sighted six or more times per pooled period (3 yr each). Nonmetric multidimensional scaling (MD), hierarchical agglomerative cluster analysis and Mantel tests were conducted to determine if any divisions were present. Mantel tests and MD plots analysis supported the delineations into the three clusters. Cluster analysis showed cluster groupings, but with less clear distinctions between the clusters. The amount and strength of associations were significantly higher within clusters than between clusters. Based on behavioral and geographic overlap, these clusters did not meet the definition of separate communities and thus were termed social clusters. These fine scale, within community divisions, are biologically and socially important aspects of their community and are crucial in understanding the dolphins’ social structure.

Environmental variations can influence the structure of ecological communities that in turn alter the grouping and association patterns of social communities. This study compares the social structure of bottlenose dolphins in the Bahamas before and after two major hurricanes. Approximately 30% of regularly seen individuals disappeared after the hurricanes, with an equal number of immigrants arriving afterwards. The primary goal of this study was to quantitatively describe social structure changes occurring after this large-scale emigration (or death) and subsequent immigration of individuals using the social analysis program, SOCPROG 2.3. The pre-hurricane results revealed one community with association patterns that were consistent with previous work on this population as well as other well-documented populations. Post-hurricane associations revealed that the community split into two distinct units, whose members associated highly within, but rarely between units. Association patterns varied between units. Immigrants assimilated well into the population, especially males. Over half of the post-hurricane associations involved immigrants, the majority between residents and immigrants, and primarily involving immigrant males. The costs/benefits of choosing to associate with an immigrant individual differ between males and females and may have been the driving force for the changes in social structure that occurred.

The social structure of coastal ecotype bottlenose dolphins, Tursiops truncatus, is largely unknown as they inhabit regions far from shore. This study reports on a community of bottlenose dolphins ≥ 27 km from Grand Bahama Island (May-September, 1993–2002). Resident and non-resident dolphins occurred in the area. Some dolphins traveled over 320 km between communities; others showed long- term site fidelity up to 17 yr. Average group size was 3–5, and was significantly larger with calves present and significantly smaller when traveling. The half-weight index was used to determine coefficients of association (COA) for individuals of known sex annually and for pooled years. Permutation tests revealed non-random associations and presence of preferred/avoided companions in all data sets. Annual COAs were low: female-female χ= 0.31, male-male χ= 0.30, and mixed-sex χ= 0.26. Mother-calf associations showed the highest values. Some males formed strong, long-term bonds. Female COAs fluctuated with reproductive status. Using pooled data, COAs were lower and the same basic trends were evident. However, strong associations seen in the annual data were not evident in pooled data. Bottlenose dolphins that inhabit offshore, shallow water show many of the same social structure characteristics as in well-studied coastal populations.